Shell and tube reheaters, wherein a first fluid enters a first or inlet section of a header and after a single, double, or multiple number of passes exits into a second or outlet header section, suffer from a variety of problems which contribute to inefficiencies and instabilities in their operation.
One such type of heat exchanger is a moisture separator reheater, used with a steam turbine to reheat moist saturated steam which is exhausted from a first turbine section before it is input to a second turbine section. In the operation of such moisture separator reheaters (MSRs) the apparatus comprises one or more reheater tube bundles disposed in series arrangement between the inlet and outlet ports of the shell and a moisture separator for removing entrained moisture from the input shellside steam as it passes into the shell. The present invention is directed to improved structure for a reheater section such as may be a portion of an MSR.
The problems encountered in MSRs, reheaters and other shell and tube heat exchangers are set forth in detail in U.S. Pat. (Application Ser. No. 890,674 filed Mar. 27, 1978 and assigned to the present assignee) to Reed et al. now U.S. Pat. No. 4,206,802, the disclosure of which is incorporated herein by reference thereto.
Briefly stated, the more serious problems with respect to MSRs and reheaters, with respect to which the invention will be discussed, although it is not limited in application thereto, relates to the subcooling of condensate from tubeside steam in certain of the reheater tubes.
Subcooled condensate tends to introduce instabilities. These instabilities stem from the condition that all tubes of the reheater involved in a given shellside steam pass are in parallel and exit to the same outlet header and, in the case in which the pressure in the outlet header may be temporarily greater than the driving force behind subcooled condensate, difficulty of draining of tubes and other attendant instabilities frequently result.
A solution to the problems described is to flush or "scavenge" the tubes of the heat exchanger with excess steam over that required to heat shellside steam. Still another technique used is to "orifice" the tube inlets to provide for different size entrance apertures for the respective tubes with the more heavily loaded tubes having the greatest aperture while the most lightly loaded tubes have the smallest entrance apertures. Since the respective tubes receive heating steam in proportion to the size of the entrance aperture, differential orificing tends to supply a greater mass flow of steam to the more heavily loaded tubes to facilitate a better distribution of steam supplied to these tubes and thus greatly reduce condensate subcooling and associated instabilities.
While the foregoing features are effective to reduce instabilities in MSR tube bundles, further changes in the shell and tube structure thereof are required to reduce the amount of scavenging steam necessary to control subcooling and thus increase the thermal efficiency of the reheaters. Additionally further means are required to further reduce and eliminate condensate subcooling and attendant instabilities.
In the aforementioned Reed et al patent the use of a high .DELTA.P thermocompressor is employed to improve the efficacy of scavenging with a given excess quality of scavenging steam. The present application achieves these and additional improvements in a less complicated manner.
Accordingly, it is an object of the invention to provide improved heat exchangers of the shell and tube type which avoid instabilities due to condensate subcooling.
Another object of the present invention is to provide improved shell and tube heat exchange reheaters which avoid condensate subcooling and associated instabilities without a loss of efficiency.
Yet another object of the invention is to accomplish the foregoing objects with a minimum of change in reheater configuration and at the least cost and with the best possible efficacy.